HVAC Mixture Control Slide Problem

I have a '97 Civic HX. The HVAC mixture control slide doesn't work
properly. It will slide to Hot or Cold but it will not stay in the middle
when the blower fan is on high.

If the blower fan is on low, it works correctly. As the blower fan speed is
increased, the slide gets more difficult to place in the middle. The wind
from the blower fan seems to be pushing on the adjustment door inside the
underdash heater box and causing the problem.

With the blower fan on high, it is impossible to get the slide to stay in
the middle. If the slide is at Hot and I start to move it toward Cold, the
slide moves easily for 1 inch then gets difficult to move. If I let go of
the slide then, the slide moves back toward Hot. If I continue to move the
slide toward Cold (with increasing force) the slide will come to about 1/2
way (between Hot and Cold), then jump 1 inch or so toward Cold. A similar
thing happens if I start at Cold and try to move the slide to the middle. I
would describe this phenomenon as a 'toggle'. The slide will stay at Hot or
stay at Cold but there is a large range in the middle where it will not
stay.

I've disassembled the dash and removed the heater box. I expected to find
something broken inside to cause this. I can't find anything broken. I
wonder if all '97 ish Honda Civics suffer from this. I understand that
'96-2000 civics are all the same. People with '96-'00 Civics; can you
comment whether or not you have experienced this problem? Any insight would
be appreciated.

Thanks,
Andy.